Determination of the exposure rate constant for 125I using a scintillation detector

A scintillation spectrometer was employed to determine the exposure rate constant for an 125I seed of the type used in therapeutic radiology. The method consisted of converting counting rate to photon fluence incident upon the detector, and then calculating the exposure rate from the photon fluence. Correlations are required for absorption of the 125I photons in the air path, in the aluminum window of the NaI crystal, and in the aluminum-oxide reflector. Corrections are also required for the escape of iodine K-characteristic radiation, and the intrinsic peak efficiency of the crystal. The experimentally determined exposure rate constant in a direction perpendicular to the long axis of the seed is 1.309 Rcm2mCi-1 h-1 +/- 5.8%, and this compares favorably with a calculated value of 1.284 R cm2mCi-1 h-1. The photon fluence as a function of angle of rotation about an axis perpendicular to the seed's long axis was also measured. These data were used to estimate an average exposure rate constant for 125I seeds of 1.089 Rcm2mC-1 h-1.